I've now fixed this. The problems occurs (I think) if your Putty client has a
default session with the same host name as the one you actually set up for
svn. I suspect plink just uses the first session with the right host name.
As the default session can't store an auto-login all sorts of havoc ensue with
the session name ultimately being sent as a command.

Modifying the Default Putty session to have a blank host fixes the problem.